Merge branch 'gitlab-runner-community' into 'master'

dockerfile: install gitlab-runner-helper from community

See merge request !3

Dockerfile

-FROM alpine:latest
+FROM alpine:edge
 
 ENV SSL_CERT_FILE /etc/ssl/cert.pem
 
-RUN echo "@edge/testing http://dl-cdn.alpinelinux.org/alpine/edge/testing" >> /etc/apk/repositories \
-    && apk upgrade -U --no-cache \
-    && apk add --no-cache git git-lfs gitlab-runner-helper@edge/testing
+RUN apk add --no-cache git git-lfs gitlab-runner-helper
 
 COPY scripts/gitlab-runner-build /usr/bin